1. Field of the Invention
The present invention relates to a new coating composition for heat-resistant substrates, in particular for coating wires, which enables exceptionally heat resistant lacquer coats to be produced and enables the coated wires to be tinned or soldered at temperatures of 370.degree. C. or above, and to the coated heat-resistant substrates, in particular coated wires, prepared therefrom.
2. Description of the Prior Art
It has been known for many years to produce coated wires capable of being tinned at comparatively low temperatures of about 375.degree. to 390.degree. C. by using wire coatings based on polyurethanes. The binders of the wire coatings used for this purpose are based on combinations of polyester polyols and phenol-blocked or alkanol-blocked polyisocyanates (DE-AS 1,170,096 or DE-AS 2,626,175). An improvement in the solderability of the coatings may be obtained by incorporating a larger number of urethane groups in the polymer film by combining blocked polyisocyanates with hydroxy functional oligourethanes (DE-OS 1,644,794).
Polyurethane wire coatings based on the above-mentioned combinations are widely used for the production of coated copper wires which are readily tinned; however, due to the fact that they only possess moderate temperature resistance (temperature index TI according to IEC 172=130.degree. to 155.degree. C., IEC=International Electrotechnical Commission), they can only be used to a limited extent. The heat classes or temperature indices of the insulating materials for coated wires are listed, for example, in "International Wire Standards-Progress Towards Harmonization" by S. Darling, in "Proceedings 19th EEI Conference", Chicago, 25th to 28th September 1989, page 56.
It is also known to coat metal wires with coatings which are much more resistant to prolonged exposure to heat (temperature index TI according to IEC 172 of about 18.degree. C.). These wire coatings are not based on polyurethanes but on heat resistant synthetic resins such as polyhydantoin (e.g., FR-PS 1,484,694 and DE-PS 2,460,206), polyamidimides (e.g., DE-OS 3,544,548, DE-OS 3,714,033 and DE-OS 3,817,614) and polyester amidimides (e.g., U.S. Pat. No. 3,652,471 and DE-PS 3,249,497). However, no wires which have been coated with wire coatings based on these synthetic resins can be tinned at temperatures below 400.degree. C.
Although polyester imides having a temperature index TI of 180.degree. C. are described as solderable (capable of being soldered or tinned) in the above mentioned publication by S. Darling, this tinning according to IEC 851 is said to be possible only at temperatures above 400.degree. C. The requirements to be met by the various types of coated wires are described in IEC 317 in which, for example, the tinning temperature for tinnable polyester imides having a temperature index of 180.degree. C. (IEC 317, Part 23) is stated to be 470.degree. C.
Polyisocyanates containing carbodiimide and/or uretoneimine groups and their use for coating wires are disclosed in EP-B-231,509. These polyisocyanates are suitable for the production of coated wires which can be soldered (Example 1 of EP-B-231,509) or for the production of heat resistant coated wires (Example 3 of EP-B-231,509), depending upon the reaction components.
Coating binders containing mixtures of very special polyamidoimides based on trimellitic acid anhydride and the above-mentioned special blocked polyisocyanates described in EP-B-231,509 are also known; according to EP-A-291,699, these binders give rise to highly heat resistant films.
The use of low molecular weight additives in binder systems based on blocked isocyanates for improving their technological properties is also known. Heat-resistant coated wires are obtained according to EP-A-287,947 by using unsaturated carboxylic acids in combination with polyisocyanates containing carbodiimide and/or uretoneimine groups. The use of N,N',N"-tris-(2-hydroxyethyl)-isocyanurate as an additive for wire coatings based on polyurethanes also results in heat-resistant coated wires (DE-OS 3,133,571).
In summary, the teachings according to the known state of the art are limited to coating wires either with wire coatings which result in highly heat-resistant coatings (temperature index TI above 180.degree. C.) or with systems which are capable of producing coated wires which can be tinned at a comparatively low temperature (about 370.degree. C.).
Therefore, it is an object of the present invention to provide a coating composition for heat-resistant substrates, in particular for wire coatings, which combines the two advantages (high heat resistance and tinning at comparatively low temperatures). Wires coated with the coating compositions according to the present invention should, in particular, be capable of being tinned at about 370.degree. C. according to IEC 851 and should at the same time have a temperature index TI according to IEC 172 of at least 180.degree. C. The other properties should to the extent possible correspond to those of known wire coatings and wires coated with such wire coatings.
It was surprisingly found that these objectives may be achieved with the coating compositions of the present invention. The invention was based on the surprising observation that the use of individual components a), b) and c) as binders was suitable for achieving the objectives stated above.